This invention relates to vulcanizing ("curing") green tires in a split mold cavity within a curing press, such as an automatic curing press, a pot heater, or other curing means. An insertable bladder is inserted in a green tire which is shaped, then cured when high temperature steam, water or other curing fluid is introduced into the bladder, this being the conventional manner in which green tires are cured.
The simple fact that the cost of producing a tire can be reduced by reducing the period of time it takes to cure a tire has sensitized tire manufacturers to the criticality of saving a fraction of a minute in the curing cycle. Over the years, the effort to minimize curing time without sacrificing quality, has been unremitting. This effort has included using higher curing temperatures, changes in rubber compounding, mechanical improvements in the curing presses, improvements in the valving, and switching from lower pressure steam to higher pressure steam. Details about such systems are referred to in U.S. Pat. Nos. 2,066,265; 3,489,833; 3,579,626; 4,027,543; 4,126,657; 4,371,483; and 4,490,325.
A typical press cycle for a passenger tire is in the range from about 10 to about 20 minutes; the sum of the times required for each of plural shaping stages is generally less than 45 secs. In a Bag-0-Matic or Autoform press, the press cycle commences with closing of the press which actuates the timer for the curing cycle; the press cycle ends when the press opens after blowdown. For economic reasons, it is desirable to shrink each period for loading, shaping, curing the tire, and blowing down the bladder, to the minimum. Saving 30 secs in the curing cycle of a passenger tire not only avoids overcuring the tire but may add up, over a period of time, to a several million dollar advantage for a manufacturer, provided of course, thus the cure can be accomplished without sacrificing the cured quality of the tire produced. To this end, the '843 patent sought to provide a perfect cure.
By "shaping" we refer to the initial inflation of the bladder to permit it to press against the inner surfaces of a radial tire uniformly and thus displace air between the bladder and the tire, and the tire and the mold; for a bias tire, it is more critical that the air between the tire and the mold be displaced. Shaping may be done in several steps. For example, the pressures may be sequentially increased from 2 psig to 5 psig, then to 10 psig, and held for a different interval of time at each pressure level. The internal pressure during the curing step may likewise be "stepped" and held at each pressure level for a different interval of time. A curing press equipped with a programmable controller ("PC") programmed to shape a tire as a function of pressure is disclosed in application Ser. No. 898,965 filed Aug. 21, 1986, which is now U.S. Pat. No. 4,744,931, the disclosure of which is incorporated by reference thereto as if fully set forth herein.
Those skilled in the art of curing tires will recognize that a tire is fully cured to desired "cured specifications" when the blowdown portion of the curing cycle is commenced, though some additional curing will continue while the tire is being blowndown, and even after it is removed from the curing press. But such additional curing, or overcuring, is not part of the essential curing covered by or within the cured specifications. Overcuring is generally undesirable, though some overcuring at the outer portions of a tire is to be expected when precisely adequate curing is provided at the point of least cure.
Presses commonly used for curing tires include Bag-O-Matic brand presses manufactured by McNeil Corp., and those known by the trademark Autoform, manufactured by NRM Corp., both of Akron, Ohio. These types of presses are automated and employ separable mold halves with center shaping and curing mechanisms using an inflatable bladder means. Each press may include a single, but usually two, simultaneously operated molds and the press is preferably individually controlled by its own PC and the necessary instrumentation and hardware which allows a press to be operated automatically. The shaping and blowdown instructions, specifically, for the PC, and the curing instructions generally, may be downloaded from a central computer each time the press cycle is to be changed.
A curing press may also be of the `pot-heater` type referred to in the '483, in which a stack of split-molds loaded with green tires is formed within a pressure vessel closed at the top, with a dome having a butt-plate against which the stack is biased by a hydraulic platform on which the stack rests. Conventional pot-heaters are of the type manufactured by United McGill of Columbus, Ohio or Pennsylvania Engineering Corp. of New Castle, Pa., inter alia.
As disclosed in the '483 patent, green tire (ambient) temperature is an essential element in the determination of the optimum number of cure equivalents or cure time for the tire. Green tire temperature is generally assumed to be the same as ambient temperature at the site of a curing press because the green tire is held in the press's loading means during the curing cycle of the preceding tire being cured. The problem of adjusting the curing cycle of a press to reflect the temperature of a green tire about to be loaded into it is a particularly serious one in those climates where wide fluctuations in temperature not only from month to month, but from day to day, cause the ambient temperature of a curing room to vary widely. A typical curing room may house a hundred, and often, up to several hundred curing presses; it is not uncommon to find the ambient temperature at the site of a press near one end of the curing room different from that at the other end by 10.degree. F. or more. This variation is routinely large during winter in the northern portions of the country, and therefore, particularly noteworthy because the green tire temperature affects the shaping and curing of the tire substantially.
In the aforementioned '843 patent, the disclosure of which is incorporated by reference thereto as if fully set forth herein, I have disclosed how to adjust the curing of a tire by tracking the location of the point of least cure. The location of this point varies depending upon a host of variables, but the precise optimum number of cure equivalents is delivered to the point so tracked, without human attention during the process. This permitted a "perfect" cure (because the precise number of cure equivalents are provided at the actual point of least cure) to cure the tire in an "elapsed cure equivalent" mode, this being preferable to operation of a curing press in an "elapsed time" mode.
The problem is that operation of a curing press in the elapsed cure equivalent mode, by tracking the point of least cure, can only be justified for those tires where the cost of failing to meet the desired cured specifications is much greater than the relatively high cost of curing the tire. Stated differently, the patented invention is particularly applicable for those tires where a "near-perfect" cure is not good enough. Without using the actual point of least cure, nor an approximated one, in the program for the PC, we get an accurate, though not precise, curing time for any tire with a particular configuration. For most passenger car tires the "near-perfect" cure is more than good enough, and the economics of providing it such a cure are good.
By "curing time" I refer specifically to the duration of the curing period. The curing period is that period between the shaping period and the blowdown period. The curing period commences with the closing of the press (upon completion of shaping), and ends with initiation of the blowdown period. The goal is neither to undercure the tire nor to overcure it any more than is dictated by the known range of variations of other factors, for example compounding of the rubber in the belts and tread, variations in construction of the tire, inter alia, the degree of which variations is approximated from experience.
A tire is undercured when it is cured for less time than is required to ensure that it is cured at the point of least core. Of course, one always makes certain a tire is never undercured, but the larger the extent of the overcuring, the poorer the cured characteristics of the tire, and the additional time used in the curing cycle adds to the cost of the cured tire. Stated differently, time not wasted in the curing cycle, not only saves overcuring the tire but also is time saved in each cycle.
The blowdown portion of the cycle is the time it takes to purge steam from the bladder in each mold cavity until the pressure is sufficiently low to open the mold. Depending on numerous factors which are detailed in patent application Ser. No. 888,896 filed July 24, 1986, which is now U.S. Pat. No. 4,721,590, the disclosure of which is incorporated by reference thereto as if fully set forth herein, this period was fixed in the prior art by a timer for each press and is about 30 sec for a passenger tire. If, for some reason, the pressure in the bladder is not low enough, the press will not open because of a pressure safety switch which mechanically responds to pressure on a diaphragm. The safety switch is set for a pressure low enough so as not to injure an operator who happens to be close by when the press opens. Such pressure is in the range from about 3 psig to about 5 psig.
By chance we recognized that a simple correlation existed between ambient temperature and the required cure time, which correlation, though unable to provide a precise optimum number of cure equivalents at a point of least cure refined for three-dimensional thermal diffusion within the tire, was accurate enough for all but the most demanding cure specificiations. We were determined to identify and refine that correlation, then utilize it in the PC. In this invention, we have done so.